1. Field of the Invention
The present invention relates to a method of fabricating a non-volatile memory device with high data retention ability, and more particularly, to a method of fabricating a non-volatile memory device to prevent diffusion of mobile hydrogen atoms and mobile ions so as to eliminate charge loss from the memory device.
2. Description of the Prior Art
In semiconductor processing, after completion of back-end processes like metallization and planarization, the main frames of the ICs are formed on the semiconductor. These ICs, after having just completed the back-end processing are easily damaged by inadvertent collision or long-term exposure to humidity and moisture. In addition, diffusion of alkali metallic ions may affect the characteristics of the IC elements. Therefore, before performing a package process, a gettering layer or a passivation layer is required to cover the semiconductor wafer, so as to protect the underlying ICs. Gettering materials, such as phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG), are most often used.
With the increasing integration and the shrinking element size of the ICs, effects of short channel, hot carriers, and diffusion of charged impurity ions and mobile ions become more rampant. Take a non-volatile memory for example, that includes an erasable and programmable read only memory (EPROM), a flash memory and an electrically erasable programmable read only memory (EEPROM), with the data retention ability of the memory cells often decreasing at high temperature. Mobile hydrogen atoms are thought to penetrate the gettering layer and diffuse into the floating gate of the non-volatile memory, thus resulting in a charge loss within the floating gate.
However, diffusion of the free hydrogen atoms may also bring advantages under some conditions. For example, hydrogen atoms combine the dangle bonds on the interface of the gate oxide layer and the silicon substrate, forming a Sixe2x80x94H covalent bond or a Sixe2x80x94OH bond. Since dangle bonds have been reduced, a threshold voltage of the MOS transistors may be adjusted as a result. It is unfortunate that Sixe2x80x94H covalent bonds or Sixe2x80x94OH bonds of the non-volatile memory device are easily broken by high-speed electrons injecting into the floating gate. As a result, hot carriers effects are produced, and hydrogen atoms may penetrate the gate oxide layer to diffuse into the floating gate.
In order to prevent mobile atoms from affecting characteristics of the MOS transistors or memory cells, Mehta et al. (U.S. Pat. No. 6,071,784) provides a method of annealing the silicon nitride layer and silicon oxy-nitride layer so as to eliminate charge loss from non-volatile memory devices. Mehta et al. forms a barrier layer or an etch stop layer to cover the MOS device. Following that, high-temperature annealing at 750xc2x0 C. is used to remove mobile atoms from the barrier layer or the etch stop layer. A dielectric layer is then formed on the barrier layer or the etch stop layer. It is, however, a disadvantage of this process to add an extra high-temperature annealing process during the back-end process of semiconductor products, so as to alter characteristics of semiconductor elements. In addition, Mehta et al. neglects the possibility that mobile hydrogen atoms and mobile ions may diffuse from a contact hole into the semiconductor elements.
It is therefore an objective of the present invention to provide a semiconductor process to eliminate charge loss and improve the data retention ability of a non-volatile memory, without the use of a high-temperature thermal process to anneal a barrier layer within the non-volatile memory.
It is another objective of the present invention to provide a method of fabricating a non-volatile memory device to effectively prevent diffusion of the mobile ions or the hydrogen atoms into the memory device.
According to this invention, a memory device is formed on a silicon substrate. A blocking layer is thereafter formed to cover a stacked gate of the memory device. A gettering layer is formed on the blocking layer followed by planarizing of the gettering layer to a predetermined thickness. A first barrier layer is then formed on the gettering layer. A contact hole is formed to penetrate through the first barrier layer, the gettering layer and the blocking layer down to the surface of the memory device. Following that, a second barrier layer is formed to cover the first barrier layer and the contact hole. Finally, portions of the second barrier layer are etched back to form a barrier spacer on the side wall of the contact hole. Therein, the first barrier layer and the barrier spacer prevent mobile atoms from vertically diffusing and laterally diffusing, into the memory device.
In brief, the present invention has advantages of:(1)effectively preventing effects of mobile atoms in the memory device, especially in the floating gate;(2)elimination of an unnecessary high-temperature annealing process; and(3)decreasing the thickness of the gettering layer so as to decrease the aspect ratio of the contact hole.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.